Conventionally, there has been known a deionized water production apparatus that passes water to be treated through an ion exchanger to perform deionization. In such a deionized water production apparatus, when the ion-exchange group of the ion exchanger is saturated to lower deionization performance, the ion-exchange group must be regenerated by chemicals (acid or alkali). Specifically, an anion or a cation adsorbed on the ion-exchange group must be replaced with H+ or OH− derived from the acid or the alkali. Recently, to remove such operational disadvantages, an electrodeionization apparatus for producing deionized water that eliminates the necessity of regeneration by chemicals has been put into practical use.
The electrodeionization apparatus for producing deionized water is an apparatus combining electrophoresis with electrodialysis. The basic configuration of a general electrodeionization apparatus for producing deionized water is as follows. That is, the electrodeionization apparatus for producing deionized water includes a deionization chamber, a pair of concentration chambers adjacent to both sides of the deionization chamber, a cathode chamber disposed outside one of the concentration chambers, and an anode chamber disposed outside the other concentration chamber. The deionization chamber includes an anion exchange membrane and a cation exchange membrane arranged oppositely to each other, and an ion exchanger (anion exchanger and/or cation exchanger) filled between these exchange membranes. Hereinafter, the electrodeionization apparatus for producing deionized water may be abbreviated to a “deionized water production apparatus”.
To produce deionized water by using the deionized water production apparatus having the aforementioned configuration, water to be treated is passed into the deionization chamber when DC voltage is applied between electrodes that are respectively arranged in the cathode chamber and the anode chamber. In the deionization chamber, an anion component (Cl−, CO32−, HCO3−, or SiO2) is captured by the anion exchanger, and a cation component (Na+, Ca2+, or Mg2+) is captured by the cation exchanger. Simultaneously, water-splitting reaction occurs at an interface between the anion exchanger and the cation exchanger in the deionization chamber to generate a hydrogen ion and a hydroxide ion (H2O→H++OH−). The ion components captured by the ion exchangers are replaced with the hydrogen ion and the hydroxide ion to be released from the ion exchangers. Released ion components are electrophoresed through the ion exchanger to the ion exchange membrane (anion exchange membrane or cation exchange membrane), and subjected to electrodialysis at the ion exchange membrane and then move into the concentration chamber. The inn components that have moved into the concentration chamber is discharged together with concentrated water flowing in the concentration chamber.
The most of the voltage that is applied to the deionized water production apparatus is used for the water-splitting reaction. Accordingly, to achieve an operation with a low voltage and a high current density, the water-splitting reaction is desirably expedited. With regard to this point, it is recognized that the water-splitting reaction in the deionization chamber can be expedited by disposing a bipolar membrane in the deionization chamber.
Patent Literature 1 describes an example of the deionized water production apparatus where the bipolar membrane is disposed in the deionization chamber. As shown in FIG. 6, the deionized water production apparatus described in Patent Literature 1 includes a pair of concentration chambers C1 and C2, and deionization chamber D provided between the pair of concentration chambers C1 and C2. In FIG. 6, a cathode chamber and an anode chamber are not shown. In deionization chamber D, single anion exchanger A and mixture M of an anion exchanger and a cation exchanger are stacked along the passing direction of water to be treated. In other words, the passing-direction upstream side of the water to be treated is filled with the anion exchanger in a single bed form, while the passing-direction downstream side is filled with the anion exchanger and the cation exchanger in a mixed bed form. Further, bipolar membrane BP is partially disposed in deionization chamber D. Specifically, in a region filled with anion exchanger A, bipolar membrane BP is disposed so that anion exchange membrane 1 can be in contact with anion exchanger A.
If ion exchanger layers having the polarity different types are stacked in the deionization chamber, the excess voltage necessary for the water-splitting reaction will vary from one layer to another and thus a biased flow of electric current causes. Hereinbelow, an ion exchanger layer including only a cation exchanger may be referred to as a “cation layer”, and an ion exchanger layer including only an anion exchanger may be referred to as an “anion layer”. An ion exchanger layer including a mixture of a cation exchanger and an anion exchanger may be referred to as a “mixed layer”.
When the cation layer, the anion layer and the mixed layer are compared with one another as regards electric resistance, electric resistances are gradually higher in the order of the cation layer, the anion layer and the mixed layer. In other words, the electric resistance of the anion layer is lower than that of the mixed layer. This means that in the deionized water production apparatus shown in FIG. 6, the anion layer lower in electric resistance than the mixed layer is in contact with the bipolar membrane. Consequently, the biased flow of electric current is more conspicuous.
It is therefore an object of the present invention to prevent a biased flow of electric current in the deionized water production apparatus, and enable an operation having a low voltage and high electric current density.